1. Field of the Invention
The present invention concerns the production of gypsum building boards.
2. Related Art
Gypsum building boards are well known and are frequently provided in surface reinforced form. Examples of such building boards include plasterboard (which is generally reinforced with paper liners) and gypsum board (which may be reinforced at or near the surface by a woven or non-woven reinforcement, or solely reinforced in its core).
Gypsum board is generally produced by shaping gypsum stucco slurry (and optionally fiber reinforcement), through an extruder or between rollers or belts. For surface reinforced gypsum board, the gypsum slurry is sandwiched between surface reinforcements before shaping. In this arrangement, the edges of the surface reinforcement or liner sheets are typically sealed together. After shaping of the gypsum board, the wet slurry is allowed to set and the set board is cut into desired lengths and dried to form the final board.
It has been found that water-swellable clays present in some natural sources of gypsum tend to increase the water demand of the slurry which ultimately forms the plaster, by forming a flocculated structure in the presence of calcium ions.
It has been proposed in U.S. Pat. No. 7,261,772 (Schwartz et al) to add to such clays a comb-branched copolymer that is formed from a polyether having terminal unsaturated groups, for example by polymerizing a polyether macromonomer having a polyalkylene oxide chain, reacted with an unsaturated monomer such as acrylic acid or methacrylic acid. The polyalkylene oxide is typically derived from ethylene oxide, propylene oxide, butylenes oxide or the like; for example a polyethylene glycol or polypropylene glycol, or random or block copolymers thereof. The above US patent suggests that certain amine compounds are necessary in the gypsum composition—specifically mentioned are aliphatic amines, quaternary amines, alkoxylated amines and amine oxides.